Currently, ASTM lists no single method that entirely encompasses the molecular weight range expected for gasolines, diesel fuels, and gas turbine fuels and/or that does not employ correlative relationships with other fuel properties to yield the average molecular weight. A number of methods for estimating average molecular weight are described in the literature. Each of the references listed in this disclosure are incorporated herein in their entirety.
For example, Espada, J. J., C. Almendros, and B. Coto, “Evaluation of Different Methodologies to Determine the Molecular Weight of Petroleum Fractions”, Energy and Fuels 25, 5076-5082 (2011) discloses a method based upon gel permeation chromatography and distillation curve. Many of the previously disclosed methods rely on empirical correlations of more readily available fuel properties to deduce the mixture averaged molecular weight. See e.g., Nelson, “Petroleum Refinery Engineering,” 4th ed., Mcgraw-Hill Book Company NY (1958); Schneider (1998), “Select the Right Hydrocarbon Molecular Weight Correlation”, Stratus Engineering, Inc, League City, Tex. (http://www.stratusengr.com/). Other methods utilize property measurements such as vapor pressure ossmometry and freezing point suppression. Such methods are designed to leverage reliable scientific theories but are primarily limited by the ability to measure very small perturbations of the specific fluid property, for example electrical conductance or fluid temperature respectively. Consequently, the expected accuracy and associated uncertainty of the reported quantity are limited. For example, ASTM D2502 and ASTM 2503 quote best case values of 5 and 14 g/mol for repeatability and reproducibility respectively.
Other so called “direct” techniques such as those involving detailed gas chromatography (GC) and gas chromatographic-mass spectrometry (GC-MS) analyses present higher accuracy for analytes which are simple mixtures, but encounter difficulties when the fluid mixture is more complex, such as is the case for real gasolines, kerosenes and diesel fuels which are commonly each composed of hundreds of individual chemical components. Such chromatographic analyses typically require considerable investment in expert use of sophisticated and expensive analytical equipment, followed by complex and time consuming interpretation of the measurements to yield an average molecular weight. The uncertainty of these techniques depends in a cumulative manner on: the proper chromatographic separation of each of the unknown components, correct identification and accurate quantification of each of the hundreds of individual molecular species that may or may not be present. In addition, the presence of the many molecular classes and isomers in these materials complicate accurate quantification by analytic GC, GC-MS) and even comprehensive multi-dimensional (e.g. two-dimensional gas chromatography-mass spectrometry (GC×GC-MS) methods, resulting in both inaccuracies and uncertainties in the determined value. Regardless of the molecular complexity of the analyte, chromatographic-based techniques are not only time consuming, and expensive, but are difficult to automate when analytes are of varying compositional character. Improved systems and a more direct, simple method for determination of the mixture average molecular weight of complex transportation fuels is desirable.